The long-term goal of the proposed research is to gain further understanding of the neural mechanisms underlying the ontogeny of coordinated behaviors. Specifically, the present study is designed to extend our understanding of the development of the neural circuitry that generates walking. This study will examine the hypothesis that the neural circuitry that is used to produce leg movements during early embryonic motility is subsequently used during hatching at the end of embryonic development and then in walking and other locomotory behaviors in the post-hatching chick. Chicks will be used for these studies because, in contrast to mammals, the embryonic stages are readily accessible to experimental manipulation and yet the relevant aspects of neural development appear to proceed similarly in chicks and mammals. In order to determine whether or not the same pattern generating circuitry is used to produce embryonic motility, hatching and walking, the leg motor output patterns typical of each behavior will first be characterized using quantitative analysis of electromyogram recordings in order to identify the similarities and differences. Then the roles of descending control, sensory input and intrinsic properties of the circuitry in accounting for the differences between the motor output patterns will be analyzed. Thus the proposed study will address the issues of 1) the role of multi-use pattern generators during development, 2) the role of sensory and descending input in modulating pattern generator output and 3) the subunit structure of pattern generating circuits. The data generated will provide insight into the neural mechanisms involved in the development of walking and will significantly enhance our understanding of the development of coordinated behaviors in general.